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This problem seems to be rooted on the fact that RawValue is a protocol type(as if you change it to e.g. String this type checks correctly), I didn't took a deep look at it just so still don't know the exact cause. Also, IgnoreAssignToDestination fix is being recorded on simplifyConformsToConstraint so a guess would be solver is trying to check a conformance where it doesn't have to ...

cc @xedin @hborla

@LucianoPAlmeida Looks like `repairFailures` is just too aggressive in this case and ends up trying to `repairByUsingRawValueOfRawRepresentableType` which is not what we want, instead we'd want to return let me go in the block that checks for presence of ValueToOptional conversion.

I have opened a PR to fix this situation - #35072

Fixed by #35072 @drodriguez Please use the next available snapshot of main branch to verify (12/15 or newer) and close this issue.

Thanks! I still haven't been able to double check the fix, since there haven't been a toolchain published since the 14th. I think the reason might be #35076 updating only one version of a test. Let's see if that can be fixed soon and I can test further.

I confirm that 2020-12-20 snapshot seems not to reproduce this bug. Thanks!

It seems that the fix might not be complete. I am reopening the issue, but tell me if you want me to open a new issue.

The following code shows the problem seems to still exist:

public enum TheEnum: RawRepresentable {
  public typealias RawValue = TheProtocol

  case case1
  case case2

  public init?(rawValue: TheProtocol) {
    self = .case1
  }

  public var rawValue: TheProtocol {
    return TheType1()
  }
}

public protocol TheProtocol {}

public struct TheType1: TheProtocol {}

public func theEnum(for input: Int) -> TheEnum {
  if input % 2 == 1 {
    return .case1
  } else {
    return .case2
  }
}

public func anotherProblem(input: Int?) -> TheType1 {
  var calculatedVariable: TheEnum? {
    if let anInput = input {
      // error: return expression of type 'TheEnum' does not conform to 'TheProtocol'
      return theEnum(for: anInput)
    } else {
      return nil
    }
  }

  // error: value of type 'TheEnum' does not conform to specified type 'TheProtocol'
  let directAssignment: TheEnum? = theEnum(for: input!)
}

Ah, looks like I got it fixed only for one situation. Going to get it updated and let you know, thank you!

Ok, I have landed #35291 which would address that problem. @drodriguez Please verify once the once snapshot is available.

I compiled the toolchain myself with that fix, and tried to compile the code that was failing for us, and seems that this second fix removes all the instances of the problem we were having. Thank you very much.

No worries! I'm going to cherry-pick this into 5.4 branch as well.